It seems a lot of people have run into the same issue using fck editor and trying to limit the characters or have a character count for it:
We use FCK Editor to implement HTML Editor. One valid point is that when entering text and using the styling, that will add extra characters to the input and the character count will be off from just the text showing. This may confuse ordinary users.
I would recommend stating that there is a max length allowed added in the editor and explain to users that they should click on the code button to see how many actual characters there are and then limit the number of characters using the validation that you already have set up.